This invention is related to apparatus for exhausting gas and fume produced during refining or smelting operations where vessels are used to reduce or purify ores or metals. It is also related to exhausting hazardous gases generated by industrial furnaces, and in particular, it is directed to a modular, tube type, off-gas hood for removing gas and fume from a basic oxygen furnace (BOF) in a steelmaking operation.
Improvements in BOF refractories and steelmaking methods such as slag splashing have extended BOF campaign life. However, campaign life is related to, and limited by, the durability of its off-gas hood system, Millet Wei, "Survey of BOF hoods in North America," Iron and Steel Engineering, Apr. 1998, pages 25-30. Off-gas hoods are a necessary part of any BOF steelmaking operation in order to insure that environmental regulations are met. When a BOF hood fails, the steelmaking operation must be shut down for hood repair to prevent the release of gas and fume into the atmosphere. In such instances, where a hood has limited durability, improvements in BOF refractories and steelmaking methods have no effect on the length of campaign life. The steelmaking operation must be shut down for hood repair regardless of the condition or maintenance schedule of the BOF steelmaking vessel. The failure rate for state-of-the-art BOF off-gas hoods ranges between 1-8 failures per year with an average 14-day shut down for each failure. Wei suggests that "Methods to reduce repair time should be explored with the goal of extending campaign life . . . . " Wei also discloses that, of the BOF steelmaking operations surveyed, 60% use tube-bar-tube (membrane) construction for their off-gas hoods, 24% of the shops use tube-to-tube construction, and 16% of the steelmakers use panel type hoods U.S. Pat. No. 3,593,974 granted to Ried).
Panel type off-gas hoods, as taught by Ried, are less expensive to manufacture than the above-two tube type hood designs (membrane and tube-to-tube). However, Wei teaches that such hood panels are associated with ". . . relatively lower water velocity higher thermal stresses and lower resistance to high pressure . . . " when compared to tube type off-gas hoods. Such conditions lead to more frequent hood failure and higher maintenance costs for panelized hoods. Additionally, panelized hoods consume a greater volume of water than the tube type hood designs.
Referring again to Wei, the membrane hood design consumes less cooling water than any other hood design known in the art, and it also generates a higher water velocity for a given water flow. Wei further teaches that tube-bar-tube hoods have an improved frequency of repair record when compared to other hood design, including the tube-to-tube designs. However, repair of any tube type hood, either membrane or tube-to-tube, is very labor intensive and time consuming. For example, in a tubular hood, the hood walls comprise long lengths of side-by-side tubing that are fastened together by welding along the lengths of the tubes. A typical hood structure comprises a conduit or flue having a geometric cross-section, for example, a circle, rectangle, trapezoid, etc. The hood flue is positioned above the mouth of a BOF vessel to collect and convey gas and fume away from the steelmaking operation. Such tubular off-gas hoods have more than 300 side-by-side welded tubes extending along the full length of the hood flue. In the event of a hood failure, due to cracking, erosion, burn through or the like, the damaged hood must be repaired to prevent gas and fume from escaping into the atmosphere, and to prevent water from flooding the steelmaking area. In the past, such off-gas hood repairs required operators to cut and splice each individual tube member extending through the damaged portion of the hood. There have been instances where as many as 80 tubes needed to be cut and splice welded to complete a single hood repair. Such extensive repair work will typically shut down steelmaking operations for about 14-days.